Microcoextruded and other light control or “iridescent” films have found a wide range of applications including aesthetic/multi-color packaging, brightness enhancing and reflective films for liquid crystal display (LCD) applications, dielectric polarizers, compensating films, mirrors (including “cold” or “hot” mirrors for allowing visible light to transmit but not infrared, or vice-versa), etc. These films are typically made by coextruding many thin layers of different polymers, and can then be used as is, or laminated/coated for various applications. For these iridescent films to work properly, the different layers typically have different refractive indices, sometimes referred to in the literature as “mismatched” refractive indices, so as to optimize reflections at each interface. It is also advantageous that the thickness of each layer be on the order of the wavelength of light or smaller (i.e., less than a few microns) so as to maximize destructive interference upon reflection. Manipulation of the actual thicknesses, layer rearrangements, refractive indices, etc., allows one to tailor reflectivity and/or optical retardation as a function of wavelength, polarization, viewing angle, and so forth. These different combinations, in turn, provide widely varying light control for the different applications described above.
A typical micro-coextruded structure, for example, might be a film containing alternating layers of a polyester (e.g., polyethylene terephthalate (PET) or polyethylene naphthalate (PEN)) and poly(methyl methacrylate) (PMMA). The polyester has a fairly high refractive index (n>1.57) and a high birefringence, typically greater than about 0.05, upon orientation. In contrast, the PMMA has a very low refractive index (n is typically less than or equal to 1.50) and very little orientation induced birefringence, thus maximizing the reflectance. The total structure might contain 50 to 100 total layers of these alternating films, which are created by a special coextrusion feedblock. Typically, the film is oriented uniaxially or biaxially, to reduce the coextruded film into the desired thickness range, and also so as to induce birefringence for applications like polarizers. Iridescent films can be made from a wide variety of polymers, not just polyester/PMMA, the only requirement being that the films have appropriate refractive indices and thicknesses for the given application. Processing issues also can play a role in the selection because processing temperatures, coextrudability, adhesion, stretching behavior, and so forth, of the polymers should generally be compatible.
One of the shortcomings of previous iridescent structures and films is their “static”, non-changeable optical properties. That is, once made, the films cannot have their reflectivity easily varied. In contrast, if the film's reflectivity could be easily changed via, for example, an applied voltage, which is the basis for the present invention, the resulting applications would be enormous. This voltage varying optical structure may be referred to as a “dynamic” light control film or an “optical modulating” film. As an example, with the appropriately applied electrodes, the film could act as a type of display element. Application of a voltage would cause the dynamic film to switch from transmission to reflection, or vice-versa, in much the same way a liquid crystal display film (LCD) works. This dynamic film, however, has the added advantage in that it may be bent/flexed around any curved surface, unlike most rigid LCDs. Other applications would include dynamically changing signs/billboards, active packaging/labeling, electrically switchable polarizing films for use in, as an example, car tinting films, greenhouses, optical modulating films for switches and waveguides, and so on. Such dynamic light control films would have tremendous opportunity for a wide range of applications.
Another film technology that is related to the present invention is that of piezoelectric films. Piezoelectricity refers to materials that generate a voltage when stress is applied, or alternatively, deform when a voltage is applied. Piezoelectric materials/films are used in a wide range of applications including electrical crystals, transducers, touchpads and screens, loudspeakers, ultrasonics, sensors, etc. There are a vast number of piezoelectric materials with the most common being quartz, various ceramics like zirconium titanate and barium titanate, and polymeric films based on poly(vinylidene fluoride) (PVDF). PVDF is commonly sold under the tradename of Kynar™ from Total Atofina and is more flexible and resilient than the brittle ceramics.
PVDF achieves its piezoelectricity only after it has been properly oriented and electrically poled to properly align the dipoles. Unoriented PVDF is in a non-polar “alpha” phase where the hydrogen and fluorine atoms are randomly arranged. In contrast, orientation causes a second randomly oriented crystal form referred to as the “beta” phase where the hydrogen and fluorine atoms are arranged on opposite sides of the chain, thus forming an electric dipole. For piezoelectric activity to occur, all of these dipoles need to be aligned in the same general direction. This is done by electrically “poling” the film at high temperature and under high electric field, to induce all of the dipoles to orient in the same direction. The sample is then quenched to lock-in this alignment. Under the application of a subsequent voltage or electric field, these aligned dipoles will try to realign relevent to the field direction, thus causing the deformation known as the piezoelectric effect. Similarly, if the film is stressed or deformed such that the dipoles are mechanically realigned, a voltage will be created across the film, which can be measured, as is the case with many piezoelectric sensors.
While there are many applications around PVDF films, most are non-piezoelectric in nature as the film is never oriented and electrically poled. Similarly, the use of such films in microcoextruded structures in a piezoelectric form (i.e., oriented and poled) is also not known. Thus, it would be desirable to incorporate piezoelectrically active films as part of a light controlling multilayer “iridescent” structure. Such a structure would be able to change its reflectivity and light controlling properties by applying an electrical voltage. The resulting dynamic film would be able to provide optical modulation in a more flexible, versatile form than, for example, traditional rigid LCDs. As stated previously, such a light controlling film would have applications in optical devices such as, for example, polarizers, optical compensators, brightness enhancing and reflective films, aesthetic films such as decorative packaging films, and “hot” and “cold” mirrors that selectively reflect only certain wavelengths, and liquid crystal displays.